The Spanish word 'espejo' (meaning 'mirror') comes from the Latin word 'speculum', which also meant 'mirror'. The Latin 'speculum' was formed by combining the verb 'specio' (meaning 'to look') with the instrumental suffix '-culum'. This combination literally created a word meaning 'an instrument for looking', which naturally came to refer to a mirror. As Latin evolved into Spanish, 'speculum' transformed into 'espejo' through regular sound changes, including the addition of an initial 'e' (a common change when Latin words beginning with 's' entered Spanish) and the simplification of '-culum' to '-jo'.

Several common Spanish words share this Latin root 'specio' (to look). For example, 'espectáculo' (show, spectacle), 'especial' (special, something worth looking at), and 'espectador' (spectator, one who looks). These words all maintain a connection to the original concept of looking or viewing.

Many English words are related to 'espejo' through the same Latin root 'specio'. The most direct cognate is 'speculum', still used in English as a medical instrument. Other common related English words include 'spectacle', 'spectator', 'spectrum', 'specimen', and 'special' - all of which relate back to the concept of looking or viewing. Even the word 'spy' is distantly related, sharing the same ancient root meaning 'to look' or 'to observe'.